A twisted pair is defined in the art of electric cable manufacture as a cable composed of a pair of insulated conductors twisted together around each other without a common covering of insulation. Each conductor is individually insulated separately prior to the twisting process. Twisted pairs are used for the transmission of electrical signals, both analog and digital, and may be used for interconnects driven in a single-ended mode or differential mode.
Signal wiring configured in a single-ended mode uses one conductor to connect the output of one device and the input of another device. If the devices being connected are high speed devices, the conductor should be a controlled impedance interconnection. This can be accomplished by placing ground wires near the signal wire or by providing an overall shield as with a coaxial cable. When a twisted pair is used for single-ended wiring, one of the conductors connects the output while the other conductor provides a return path usually referred to as ground. For this type of interconnect, it is not important that the two conductors of the pair be of equal electrical length.
High performance digital systems frequently use differentially-driven signals. Differential signals provide more precise timing, better noise immunity, and higher signal fidelity. With differential wiring the true signal and its logical complement are sent to the receiving device. The receiving device measures the differential voltage between the two signal lines. As with single-ended systems, if the digital signals are coming from high speed devices the signal wires must be placed in a controlled-impedance environment. There are several advantages to using this type of interconnect. In a twisted pair configuration the signal lines are run in close proximity to each other. Since the signals will be switching directly opposite to one another, the fields generated by these switching signals will cancel out, thereby generating very little electro-magnetic radiation. For the same reason, any radiated electro-magnetic fields impinging on this interconnect system will be sensed equally by both lines, and since the receiving device is only measuring the differential voltage between the two signals, this common-mode noise will be ignored by the receiver. A disadvantage of differentially driven interconnects is that now two conductors are required for each signal. A twisted pair construction is ideally suited to differentially-driven signal transmission. To assure proper operation, each conductor must be well matched for characteristic impedance and time delay.
If the time delays of the two conductors are not well matched, the true signal and logical complement will be received at different times, thereby limiting the useful physical length of interconnect which can be used before signal fidelity is compromised. For the time delays of the two conductors to be well matched, two conditions must be met simultaneously. First, each conductor must have the same dielectric constant, E, since the velocity of propagation (Vp), and hence time delay, of the conductor is determined by the relation: Vp: 1/.sqroot.E. Secondly, the physical lengths of the two conductors must be matched.
It has been observed that it has become more difficult to meet the requirements needed for high-speed digital differentially-driven systems with twisted pair cables, partly because it is not easy to obtain insulated conductors having exactly equal length and equal signal transmission speed end to end. If one unreels a length of twisted pair cable manufactured by the several methods known in the art and cuts a length of cable from the reel, the signal conductors therein are found to be not of equal length owing to the stresses and movements of the insulated wires as the twisted cable is formed.
The present invention provides a process for preparing twisted pair cables having insulated conductors of equal physical and electrical length.